A process of the same kind is known from “Melamine and Guanamines”, section 4.1.3 of Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2001 Electronic Release. Furthermore, in the known process, the mixture, prior to being supplied to the absorption section, is subjected to a partial condensation step.
The mixture in the known process contains, as mentioned above, NH3, CO2 and H2O. The CO2 and a part of the NH3 in the mixture mainly originate from the reaction of urea to form melamine, which proceeds as follows:6 CO(NH2)2-->C3N6H6+6NH3+3CO2
Usually NH3 is used as fluisidation gas and atomizing gas during the melamine synthesis, which may also cause NH3 to enter the mixture. Fluidization gas serves to keep the synthesis catalyst in a fluid state. Atomizing gas serves to atomize the urea in the synthesis reactor. The H2O in the mixture mainly originates from the first cooling step, where an aqueous flow is used as a cooling medium for the reaction product; here the H2O partly evaporates.
The aforementioned known absorption section, which is designed as a column, is intended amongst other things to separate a flow of gaseous NH3 from the mixture and discharge it from the absorption section. The discharged flow of gaseous NH3 is removed and generally returned to a melamine synthesis reactor to serve there as a fluidization gas and/or atomizing gas. A part of the gaseous NH3 is passed through a scrubber, whose object it is to to separate and discharge so-called inert compounds such as H2 and N2.
The known absorption section is also intended to form, from NH3 and from virtually all CO2 and H2O from the mixture, an ammonium carbamate solution and discharge that solution. The discharged ammonium carbamate solution is removed, after which generally in downstream steps the CO2 and the NH3 are liberated from the discharged ammonium carbamate solution, either as such or in the form of an ammonium carbamate solution with a considerably reduced water content, in which process a practically pure flow of water is released which in general is a waste stream and is discharged. As is known, the urea synthesis efficiency decreases as the quantity of water in the raw materials increases. The liberated CO2 and a part of the liberated NH3, as such or in the form of an ammonium carbamate solution with a considerably reduced water content, can serve as a raw material for the production of urea or for the production of ammonium nitrate or ammonium sulfate. The remaining part of the liberated NH3 is liquefied in the known process and recirculated to the absorption section as part of the flow of liquid NH3.
A disadvantage of the known process is that the recovery of the NH3 to be recirculated to the absorption section from the ammonium carbamate solution requires a great deal of energy, for example in the form of steam.
The object of the invention is to reduce the energy consumption in the downstream steps that are applied to the discharged ammonium carbamate solution.
The said object is achieved in that a part of the discharged ammonium carbamate solution is returned to the first cooling step and a part of the discharged gaseous NH3 is liquefied in a second cooling step and returned to the absorption section.
The process according to the invention ensures that less NH3 is removed via the ammonium carbamate solution per unit time than in the known process. As a result, the quantity of NH3 which is liberated in the downstream steps for recirculation to the absorption section can decrease, which yields an energy saving, for example in steam consumption. A further advantage of the process according to the invention is that less H2O is removed per unit time via the ammonium carbamate solution than in the known process. This allows a decrease in the liquid load of the equipment items in which the downstream steps are carried out for liberating the CO2 and the NH3 from the removed ammonium carbamate solution, for example for the preparation of urea. As a result said downstream steps can be carried out in smaller equipment items and with less energy consumption than in the known process, which is cheaper.
The process according to the invention comprises a first cooling step for cooling an essentially gaseous flow originating from a melamine synthesis reactor and containing essentially melamine, NH3 and CO2. The flow from the melamine synthesis reactor is contacted with a coolant. The coolant comprises a flow consisting essentially of an ammonium carbamate solution to be discussed later. The coolant comprises in addition generally also a flow essentially consisting of water; for this purpose use is usually made of an aqueous flow originating from a recovery section. The coolant flows can be supplied separately or jointly to the first cooling step. As the flow from the melamine synthesis reactor is contacted with the coolant there evolve a solution (containing dissolved melamine in an aqueous phase) or a slurry (containing melamine crystals in an aqueous phase) and a generally gaseous mixture containing NH3, CO2 and H2O. The solution or the slurry is discharged from the first cooling step and fed to a recovery section in which melamine crystals are formed (if necessary) and separated. The said mixture is usually gaseous and usually contains more than 40 wt % NH3, less than 50 wt % CO2 and less than 40 wt % H2O at a pressure usually between 0.1 MPa and 4 MPa, and is fed to an absorption section. In addition a flow of liquid NH3 is also fed to the absorption section. The percentages stated here and hereafter are percentages by weight, unless otherwise indicated.
In the absorption section according to the invention the mixture is contacted with the flow of liquid NH3. This gives rise to a flow of gaseous NH3 and virtually all CO2 and H2O go into the liquid phase. To facilitate the transition of CO2 into the liquid phase, an additional flow consisting essentially of water may be fed to the absorption section. The required quantities of the flow of liquid NH3 and of the additional flow consisting essentially of water that are fed to the absorption section are largely determined by the desired purity of the discharged flow of gaseous NH3. Usually a purity of 99% or higher is desired. The higher the desired purity of the discharged gaseous NH3, the larger the required quantities of the flow of liquid NH3 and of the additional flow consisting essentially of water.
The flow of gaseous NH3, consisting essentially of NH3 and which in addition may contain up to approximately 1 wt % of other compounds such as CO2, H2O, N2 and H2, is discharged from the absorption section. In the process according to the invention a part of the discharged gaseous NH3 is liquefied in the second cooling step and returned to the absorption section; the remaining part is removed and, as previously indicated, usually used as fluidization gas and/or atomizing gas in the melamine synthesis reactor. It is an advantage of the process according to the invention that the gaseous NH3 can be used directly as fluidization gas an/or atomising gas, without the need of an evaporating step as is the case when the NH3 to be used is supplied in liquid form. Liquefication in the second cooling step may be accomplished by means known per se, such as for example by means of a heat exchanger. An advantage of the process according to the invention is that the quantity of fresh liquid NH3 to be fed to the absorption section can decrease, because a flow of liquid NH3 is already available. A further advantage of the process according to the invention is that inert compounds such as for example N2 and H2, which cannot be easily liquefied because their condensation temperature is much lower than that of NH3, can readily be separated from the liquefied NH3 and removed as an inert gaseous vent stream. The inert vent stream will in general still contain a residual quantity of NH3. If it is desired to remove this residual quantity of NH3 the inert vent stream can be passed through a scrubber, which can however be significantly smaller than the aforementioned scrubber in the known process.
The liquid phase formed in and discharged from the absorption section consists essentially of an ammonium carbamate solution. By this is meant a solution of ammonium carbamate in water, which in addition may contain other compounds such as free dissolved NH3, free dissolved CO2 and ammonium bicarbonate. In the process according to the invention a part of the discharged ammonium carbamate solution is returned to the first cooling step; the remaining part is removed. The returned ammonium carbamate solution serves as a coolant in the first cooling step, will largely evaporate into NH3, CO2 and H2O and will thus be absorbed in the mixture which is released in the first cooling step and is fed to the absorption section; in this way there is formed in the process according to the invention a circulation flow between the first cooling step and the absorption section.
The removed ammonium carbamate solution is generally the most important medium for removing the CO2 evolving in the urea-to-melamine reaction. As a result, the quantity of CO2, as such or as an ion, in the removed ammonium carbamate solution is usually an important control parameter for operating an absorption section in a process for the preparation of melamine and also for operating the absorption section according to the invention. The part of the discharged ammonium carbamate solution that is returned to the first cooling step is determined preferably in relation to the quantity of CO2 that is present in the removed ammonium carbamate solution either as such or as an ion. The quantity of CO2 in the removed ammonium carbamate solution is hereinafter referred to as removed CO2. The quantity by weight of ammonium carbamate solution which is returned to the first cooling step divided by the quantity by weight of removed CO2 is preferably between 0.01 and 5, more preferably between 0.3 and 2.0 and most preferably between 0.7 and 1.7.
The part of the discharged gaseous NH3 which is liquefied can also be expressed in relation to the quantity of removed CO2. The quantity by weight of discharged gaseous NH3 which is liquefied divided by the quantity by weight of removed CO2 is preferably between 0,01 and 5, more preferably between 0.1 and 2.0 and most preferably between 0.5 and 1.5.
The absorption section may be of any known design, such as for example a trayed column so configured that the formed ammonium carbamate solution and the formed flow of gaseous NH3 can be discharged separately.
In order to form the ammonium carbamate solution in the absorption section it is generally necessary to withdraw heat from the mixture. Therefore, prior to being fed to the absorption section, it is preferred to subject the mixture to a third cooling step, wherein partial condensation takes place whereby a liquid phase and a gas phase are formed. The liquid phase then consists essentially of an ammonium carbamate solution. In a further preferred embodiment the liquid phase is separated from the mixture. It is then possible to remove this liquid phase separately from the ammonium carbamate solution coming from the absorption section or to wholly or partially return it to the first cooling step. An advantage of the performance of the said third cooling step is that the operation of this step appears to be a control tool in determining the quantity of gaseous NH3 that is released from the absorption section as a consequence of the fact that not all compounds of the mixture condense proprotionately; particularly H2O condenses preferentially, which makes it easier to separate gaseous NH3 in the absorption section.
In another preferred embodiment of the process according to the invention a compression step is carried out on least the part of the discharged gaseous NH3 that needs to be liquefied in the second cooling step. This presents the advantage that the condensation temperature of the gaseous NH3 rises, which simplifies condensation. Preferably the gaseous NH3 pressure is increased to at least 1.5 MPa, more preferably 1.8 MPa or even higher, for example 2 MPa or more. This presents the advantage that the condensation temperature of NH3 at the stated pressures exceeds the temperature level at which plant cooling water circuits usually operate. This preferred embodiment can readily be combined with the aforesaid preferred embodiments such as the application of partial condensation.
The ammonium carbamate solution being formed in the absorption section has an amount of NH3 and NH3-derived compounds such as ammonium ions, and an amount of CO2 and CO2-derived compounds such as carbamate- or carbonate ions. The weight ratio between these two amounts is expressed as the N/C ratio of the ammonium carbamate solution. In a further embodiment of the process according to the invention, the N/C ratio in the ammonium carbamate solution is reduced to 1.4 or less by returning a part of the discharged ammonium carbamate solution to the first cooling step and by subjecting the gaseous NH3 and/or the mixture to a second, respectively third cooling step. Preferably, the N/C ration is reduced to 1.3 or less, more preferably to 1.25 or less, most preferably to 1.2 or less. In this embodiment, it is preferred that the third cooling step comprises a partial condensation step on the mixture as describer above, prior to the mixture being fed to the absorption section, with a liquid phase and a gas phase being formed. Preferably, the liquid phase is subsequently separated from the mixture. Advantageously, a part of the discharged gaseous NH3 is liquefied in the second step and returned to the absorption section, as described above. Here, it is preferred as described above that a compression step is carried out on at least the part of the discharged gaseous NH3 that is to be liquefied.
The first digit of the numbers in the figures is the same as the number of the figure. Where the last two digits of the numbers of different figures are the same, they refer to the same item.